1. Field of the Invention
The present invention relates to a transistor array for use in a liquid crystal display, an organic EL display, an x-ray sensor, a memory and other electronic apparatuses, and also relates to an active-matrix substrate including such a transistor array and a display device including the active-matrix substrate. The present invention further relates to a jig assembly used for making the transistor array and a method of fabricating the transistor array.
2. Description of the Related Art
An active-matrix-addressed liquid crystal display (LCD), which uses thin-film transistors (TFTs) as its switching elements, achieves a display quality which is at least comparable to, and often superior to, that of the conventional cathode-ray tube (CRT) display. Also, an active-matrix-addressed LCD is a thin and lightweight display device that realizes a high resolution with significantly reduced power dissipation. Furthermore, an active-matrix-addressed LCD is also implementable as a display device with a huge screen. Having all of these advantageous features, the active-matrix-addressed LCD is currently being popularized rapidly in various fields of applications and is expected to be a next-generation image display device that replaces the conventional CRT in the very near future. Meanwhile, an organic EL display, which uses an organic electroluminescent material as a display medium, has also attracted much attention in the art recently and is also considered to be among potential next-generation image display devices.
Recently, these next-generation image display devices are not just used as alternatives to the conventional CRTs but are also the objects of vigorous research and development to realize so-called “electronic paper”. That is to say, these display devices are currently under modification so as to provide replacements for conventional paper-printed materials. For that purpose, those image display devices should be made as “flexible displays”, which never experience failure even when folded or rolled and which are readily portable at any time.
To make such a flexible display by conventional manufacturing technologies that have been applied to fabricate an LCD or an organic EL display, glass, which has been used extensively as a substrate material, needs to be replaced with some elastic substrate material that is deformable even at room temperature (e.g., plastic or stainless steel). However, such an elastic material poorly resists the heat. For example, when exposed to intense heat, a plastic substrate may deteriorate, emit a toxic gas, or be deformed or warped excessively. Also, even if a heat-resistant plastic or stainless steel is used as a substrate material, the substrate made of such a material is also deformed or warped due to a non-negligible difference in thermal expansion coefficients between that substrate and a thin film to be deposited on the substrate. Accordingly, to avoid these unwanted situations, a substrate made of such a poorly heat resistant material should not be exposed to heat in excess of about 200° C.
However, some of the conventional manufacturing and processing steps that have been carried out to fabricate an image display device require a processing temperature that exceeds about 200° C. For example, in the manufacturing and processing step of making an amorphous silicon TFT, which is used as a switching element for an image display device, the gate insulating film and amorphous silicon film of the TFT are normally formed at temperatures exceeding about 300° C. Accordingly, if the heat resistant plastic or stainless steel is used as a substrate material, the conventional manufacturing process cannot be used as it is.
Stated otherwise, the plastic or stainless steel may be used as a substrate material for an image display device if the gate insulating film and amorphous silicon film of the amorphous silicon TFT can be formed at temperatures of less than about 200° C. In that case, however, a high quality gate insulating film or amorphous silicon film (e.g., a film exhibiting good dielectric strength, in particular) is very hard to obtain. Also, when a TFT is made of those poor quality films, the threshold value of the TFT will change significantly after long hours of operation.
Recently, an LCD that uses polysilicon TFTs as its switching elements has been researched and developed vigorously. An image display device of this type is partly characterized by providing a circuit for controlling a drive signal for the display device on a glass substrate. Thus, compared to the LCD that uses amorphous silicon TFTs as its switching elements, the LCD including the polysilicon TFTs achieves a higher resolution.
In an LCD including polysilicon TFTs, however, the driver thereof (e.g., a CMOS inverter) needs to be made of a polysilicon with an electron mobility typically exceeding about 100 cm2/V·s. It is not easy to deposit such a polysilicon on a glass substrate. For example, the silicon on the glass substrate should be fused with the temperature of the glass substrate maintained at about 600° C. or less by some special technique such as a laser annealing process. Accordingly, it is difficult to apply the technique requiring such a high-temperature process to an image display device that uses plastic or stainless steel as a substrate material.
To overcome these problems, Japanese Laid-Open Publication No. 10-91097 discloses a display device including a string of transistors. In the transistor string, a number of transistors are arranged along the length of a conductor core on which an insulating film, a silicon film and an n+-type ohmic contact layer are stacked in this order. In this technique, the gate insulating film and the silicon film that require high-temperature processing are included in the transistor string, which is bonded onto a substrate after those films have been stacked on the conductor core. Thus, the substrate is never exposed to heat. For that reason, a substrate having a low distortion point (e.g., a plastic substrate) may be used.
Japanese Laid-Open Publication No. 9-203910 also discloses a similar technique of making a display device by using a string of transistors that are arranged along the length of a metal core. FIGS. 63A and 63B show one of the TFTs 501 disclosed in Japanese Laid-Open Publication No. 9-203910. As shown in FIGS. 63A and 63B, the TFT 501 includes a metal core 502, an insulating film 503 that covers the metal core 502, an amorphous silicon layer 504 that covers the insulating film 503, and a source electrode 506 and a drain electrode 505 on the amorphous silicon layer 504. Although not shown in FIGS. 63A or 63B, a number of such TFTs 501 are arranged along the length of the metal core 502.
FIG. 64 illustrates how the TFT 501 shown in FIGS. 63A and 63B may be used as a switching element for an active-matrix substrate. As shown in FIG. 64, a pixel electrode 508 on the back surface of a substrate 507 is connected to an electrode 509a by a metal connecting member, and the electrode 509a is electrically in contact with the drain electrode 505 of the TFT 501. Also, a source line 510 having an electrode 509b is arranged so as to cross, and be electrically connected to, the source electrode 506 of the TFT 501.
In the active-matrix substrate shown in FIG. 64, the transistor string needs to be disposed such that the electrode 509a on the substrate 507 is accurately aligned with the drain electrode 505 of the TFT 501. To make an active-matrix substrate, normally hundreds of transistor strings need to be arranged. Also, each of those transistor strings includes hundreds of TFTs 501. Accordingly, in each of those hundreds of TFTs 501 included in a single transistor string, the drain electrode 505 thereof needs to be accurately aligned with its associated electrode 509a. It is also necessary to accurately align the electrode 509b of the source line 510 with the source electrode 506 of each TFT 501.
To carry out these alignments, the transistor strings and the source lines 510 need to be arranged accurately with respect to the substrate 507. Thus, the positioning accuracy to be achieved in such an alignment process is far inferior to the accuracy to be achieved by the conventional photolithographic process. For that reason, it is very difficult to prepare an active-matrix substrate by the conventional method described above.